Atrial fibrillation is a common cardiac arrhythmia. Although it is not life threatening, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. Symptoms of atrial fibrillation may include heart palpitations, dizziness, and even loss of consciousness.
Atrial fibrillation can occur suddenly. It is caused by chaotic activity of the atria of the heart. The chaotic atrial activity in turn causes the ventricular activity to be disassociated from the atrial activity. The ventricular activity becomes rapid and variable.
To terminate atrial fibrillation, it is often necessary to defibrillate the atria. This entails the delivery to the heart of a brief electrical shock. The shock may be applied externally through two electrodes placed on the chest or directly by an implantable device through implanted electrodes.
When atrial fibrillation is terminated by an electrical shock, it is most desirable to deliver the shock at a time which avoids the vulnerable period of the ventricles. The vulnerable period is that time within a cardiac cycle wherein myocardial tissue is prone to develop tachycardia or fibrillation when stimulated, even with a low magnitude electrical pulse. The vulnerable period of the ventricle is approximately represented on an electrocardiogram by the top of the T-wave. The T-wave represents the time in which the ventricular myocardium repolarizes following a ventricular depolarization (R-wave). To be safe, it is generally considered prudent to not deliver any shock to the heart during a T-wave.
The key then to defibrillating the atria is to avoid the T-wave of the ventricles. However, avoidance of the T-wave requires that the time of the T-wave be accurately known. This is difficult in the atrial fibrillation environment where the ventricular activity is erratic. During atrial fibrillation, the ventricular rate is generally elevated and irregular. Hence, predicting with any certainty as to when a T-wave will begin or end in an effort to safely terminate atrial fibrillation with a defibrillating shock would be clinically impossible.